Recycling of plastic containers has become increasingly more popular over the last several years. However, while the plastic containers are recyclable, the materials used to make the shrink wrap labels applied to the containers are often not recyclable. Therefore, it is necessary to remove the shrink wrap label from a plastic container, or separate the label material from the container material, to allow recycling of the plastic container. When the labels are not easily separable from the containers, many containers are not recycled that otherwise would be, and recycling yields are reduced. Thus, there is a need to produce easily separable labels to increase the efficiency of recycling.
The Association of Plastics Recyclers, guidelines from brand owners, and publications of the National Association for PET Container Resources (NAPCOR), are all pointing to the issue created by full body sleeve labels in the recycling system. The sleeves tend to obscure the automated detection systems during the sorting process. As a result, PET recycling facilities have seen decreasing yields as sleeved bottles tend to end up being separated from clear PET containers and discarded.
To alleviate the problems with sorting, recyclers and brand owners tend to recommend paper labels, or partial sleeves made with low density thermoplastic materials, such as polyolefins. However, such materials do not have the desirable shrink properties or printability of higher density substrates such as PET-G.
Moreover, there are currently certain restrictions when applying shrink wrap labels wherein the labels must be sealed at a seam to ensure a strong bond at the seam. For example, it is necessary to avoid printing in the margins of a shrink wrap structure prior to adhering or solvent-welding them together. In addition, it is necessary to clean or wipe the margins of the shrink wrap structure in the area to be seamed prior to adhering or solvent-welding them together.
Shrink sleeve separation from PET containers is typically achieved in the sink/floatation tanks as part of the overall PET recycling process, starting from bales of PET containers and ending with clear recycled PET (R-PET) flakes to be re-used for container making. Several film/resin suppliers, including Exxon, Toppas, and Cryovac, or converters such as Fujiseal, have introduced low density films or co-extruded film structures with low density (below 1.0 or at least below 1.05), to address the shrink sleeve separation issue. Such films are generally based on mono- or multi-layer structures including polymers built around a low density core, such as polyolefins, or micro-voided/cavitated polymer, optionally wrapped by a PET-G skin, and, if needed, a tie layer between the core and the PET-G. The overall density of the film is designed to make it floatable in the sink/floatation tank. However, if the density prior to printing is not low enough, it is possible for the printed sleeve to become too dense, and therefore unable to be separated from the standard PET to be recycled.
PET-G is the preferred material for shrink wrap labels. However, PET-G has a higher density (about 1.3), which hinders separation in the sink/floatation tanks. The density of PET-G may be reduced if voids are created in the extruded film. Incorporation of a blowing agent into a coating offers the advantage that the overall label structure can be significantly reduced in density after activation of the blowing agent by various means, and separated from the container by floatation in a suitable liquid. In principle, this will work both for an entire label, and a ground up mixture of label and container. A coating will show an advantage over an ink for two reasons. First, coating of the entire label means that the blowing agent will be evenly distributed over the label, so that no one area has a higher concentration of blowing agent. Conversely, when the blowing agent is incorporated into the ink printed only in selected areas, those areas will have a higher concentration of blowing agent. Because the blowing agent in a coating is more evenly distributed, it means that a smaller particle size and/or lower coat weight of blowing agent can be used, while still achieving the desired level of density reduction. This, in turn, means that a higher level of transparency, gloss, and slip can be maintained in the overall shrink label structure.
Second, a 100%, even, coating of an entire label will give a consistent level of density reduction between different label designs, compared to an ink which may be printed at a wide range of percentage coverages. This simplifies and standardizes the recycling process.
Additionally, a micro-voided or cavitated film is opaque white, while a foamable coating approach may provide transparency or translucency. Blowing agents that generate a gas on exposure to heat, pH change, photonic radiation and/or thermal energy may comprise any of the types that are well-known to those of skill in the art (see, for example, http://en.wikipedia.org/wiki/Blowing agent and http://www.eiwa-chem.co.jp/en/product/types.html). Blowing agents may also include metal carbonates and bicarbonates, azides, azodicarbonamide, and diazonium salts.
In addition, it is well known to those of skill in the art that inks can bleed through from their desired location, either into an undesired location on a printed article, or even onto the body parts of someone holding that article (which may be aided by a body fluid such as sweat or saliva). An appropriate coating may alleviate the problem of bleeding.
A coating that is a gas barrier may also be advantageous. Gas barrier coatings are known in the art. However, there are no known coatings that are gas barriers combined with the ability to work as “de-seamable” coatings to improve recycling.
Thus, there is currently no practical or economical solution built around printable materials to address the shrink sleeve issue in the PET recycling process. Manufacturers either must use less-preferred substrates that are less dense than PET-G, or move to more complex, and likely more expensive, floatable film structures. Therefore, there is a need to provide a method to produce labels that have good bond strength at the seams, while still being easily separable from the plastic container during the recycling process.